Circuits and methods for current measurements referred to a precision impedance

ABSTRACT

Circuitry and methods for obtaining accurate measurements of current supplied by an integrated circuit are provided. Current calculations are performed using information from a precision termination resistor and from the ratio relationship of two on-chip resistors. The invention provides a way to obtain accurate current measurements without the use of component trimming.

BACKGROUND OF THE INVENTION

This invention relates to driver circuitry. More particularly, thisinvention relates to driver circuitry that performs current measurementsusing a precision external resistor.

In the past, the semiconductor industry has utilized variousconfigurations of “driver circuitry” for supplying power to loads thatare external to an integrated circuit. Common examples of such externalloads include transmission lines, communication systems, electricmotors, and illumination systems. One characteristic of driver circuitrythat is of interest to system designers is the amount of current thedriver circuitry actually supplies to the external load. In certainapplications, such as those involving power transmission or lightingsystems, it is not necessary for the driver circuitry to obtain precisevalues of the supplied current. Other applications, however, such ascommunications systems, often rely on precise bias and modulationcurrent measurements to properly function. Previously, precision currentmeasurements have been made possible by “trimming” on-chip resistors andvoltage references to obtain tolerance values suitable for currentsensing. This method, however, is costly and time consuming.

In light of the foregoing, it would therefore be desirable to providecircuits and methods for accurately calculating the current supplied bydriver circuitry without resorting to costly and time consuming trimmingtechniques.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide circuitsand methods for precisely calculating the current supplied by drivercircuitry without resorting to costly and time consuming resistortrimming techniques.

This and other objects are accomplished in accordance with theprinciples of the present invention by providing circuits and methodsfor precisely calculating the current supplied by driver circuitrywithout resorting to costly and time consuming resistor trimmingtechniques. Current calculations are performed using information from aprecision termination resistor and from the ratio relationship of twoon-chip resistors. The invention provides a way to obtain accuratecurrent measurements without the use of component trimming.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention willbe apparent upon consideration of the following detailed description,taken in conjunction with the accompanying drawings, in which likereference characters refer to like parts throughout, and in which:

FIG. 1 is a schematic diagram of a driver circuit constructed inaccordance with the principles of the present invention.

FIG. 2 is a schematic diagram of another driver circuit constructed inaccordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is an illustrative block diagram of a driver circuit 100constructed in accordance with the principles of the present inventionthat can be used to obtain accurate measurements of supplied current.Driver circuit 100 includes a sourcing circuit 10, a sense resistor 15,an external termination resistor 25, analog to digital converters 40 and55, a sinking circuit 45, and a modulation resistor 50.

Transmission line 30 and load 35 represent the external load driven bycircuit 100. Capacitor 20 may also be included in circuit 100, ifdesired. Although other arrangements are possible, the components withindotted line 60 are typically constructed on an integrated circuitdevice.

As shown in FIG. 1, sourcing circuit 10 provides a substantiallyconstant source current (I_(S)) to sense resistor 15, terminationresistor 25, and load 35 (through transmission line 30). Sourcingcircuit 10 may be any circuit configuration suitable for providing asubstantially constant current such as current mirror type biascircuitry.

The current supplied to load 35 may be varied by periodically switchingsinking circuit 45 ON and OFF, allowing a modulation current (I_(M)) topass through it during an ON state, and acting as an open circuit duringan OFF state. In some embodiments, sinking circuit 45 may be configuredto turn ON and OFF partially to improve response time. Sinking circuit45 may include any circuitry suitable for switching between ON and OFFstates such as a transistor or armature type switch.

The value of modulation resistor 50 affects the operation of circuit 100when sinking circuit 45 is switched ON. For example, when sinkingcircuit 45 turns ON and current flows across modulation resistor 50, theamount of current supplied to load 35 decreases in proportion to theresulting current divider network. Thus, if modulation resistor 50 has avalue significantly less than load 35, the amount of current supplied toload 35 (I_(L)) decreases significantly. On the other hand, if the valueof modulation resistor 50 is much greater than load 35 (i.e., a factorof 10 or more), load current I_(L) will only decrease somewhat. Thus, acircuit designer may select an approximate value for modulation resistor50 with respect to load 35 to control the current differential appliedacross it. This allows circuit designers to define the range of currentvalues supplied by circuit 100, such as those associated with thetransmission of logic “high” or a logic “low,” signal to load 35.

Driver circuit 100 will operate without termination resistor 25.However, because transmission line 30 typically has a characteristicimpedance similar to, but not perfectly matched with load 35, a portionof the transmitted signal is reflected back to the driver circuit,causing signal distortion. This problem may be corrected, however, bythe addition of termination resistor 25 and capacitor 20, which allowscurrent flow when the line voltage changes. These components help toabsorb the reflected energy from load 35 so that high speed currentcannot circulate back into the load. The present invention takesadvantage of the fact that termination resistor 25 typically hassignificantly higher tolerance level than those found on an integratedcircuit, usually around ±1%, which can be used to obtain accuratecurrent measurements.

Measurement of source current I_(S) and modulation current I_(M) isperformed using analog-to-digital converters (A/D converters) 40 and 55.This is accomplished by measuring the voltage drop across terminationresistor 25, source resistor 15, and modulation resistor 50. The voltagedrop across resistor 50 is divided by the voltage drop across resistor15 and the voltage drop across termination resistor 25 divided by areference voltage (not shown). These operations produce discrete values.In FIG. 1, these discrete values are depicted as ‘data’ outputs that aresent from A/D converters 40 and 55 to computational circuitry orsoftware (not shown) that performs the mathematical operations necessaryto determine the modulation current I_(M) and source current I_(S)(discussed in more detail below). Such circuitry may include, but is notlimited to, a multiplication amplifier, a microprocessor, a programmablelogic array, etc.

To calculate modulation current I_(M), the value of source current I_(S)must be known. Generally speaking, source current I_(S) is substantiallyequal to the current across termination resistor 25 (I_(T)) when nocurrent flows through capacitor 20. Current will only flow throughcapacitor 20 when the voltage across it changes. Because this occursonly for a very short duration at the instant sourcing circuit 45switches from ON to OFF (and vice versa), an accurate measurement ofsource current I_(S) may be obtained by measuring the voltage dropacross precision termination resistor 25.

A measurement of the termination voltage (i.e, the voltage drop acrosstermination resistor 25) may be used as a first step in determining thevalue of source current I_(S). For example, during operation, A/Dconverter 40 may take an analog voltage measurement of the terminationvoltage with respect to a reference voltage (not shown). Thismeasurement, which is represented as X_(TR), is the termination voltagedivided by the reference voltage. Accordingly, the variable X_(TR) is adimensionless ratio of these two voltages, having a value between 1 and0 such that:X _(TR) =V _(T) /V _(REF)  (1)

Furthermore, it will be appreciated that the termination voltage isequal to the product of the termination current (I_(T)) and thetermination resistance (R_(T)). Substituting this relationship intoequation (1) gives the following:X _(TR)=(I _(T) *R _(T))/V _(REF)  (2)

Because source current I_(S) is equivalent to termination current I_(T),equation (2) may be re-written as:X _(TR)=(I _(S) *R _(T))/V _(REF)  (3)

Solving equation (3) for source current I_(S) gives the following:I _(S) =I _(T) =X _(TR) *V _(REF) /R _(T)  (4)

Thus, as demonstrated above, circuit 100 may calculate the sourcingcurrent I_(S) as a function of precision termination resistor 25. Insome embodiments of the present invention, equation 4 may be solved byan external computing device. The input to such a device is obtainedfrom A/D converter 40.

It will be appreciated from the above that the accuracy of the sourcingcurrent calculation is dependent only upon precision terminationresistor 25 and an untrimmed internal voltage reference (not shown).Typically, these circuit components have tolerances of ±1% and ±5%,respectively. Thus, using the following well-known statistical errorequation:E=√{square root over (X ² +Y ²)}  (5)where X represents the tolerance of termination resistor 25, and Yrepresents the tolerance of the voltage reference, it can be seen theaccuracy of the source current measurement is approximately ±5%. This isa dramatic improvement over conventional systems that rely onmeasurements based on an untrimmed voltage reference and an untrimmedsense resistor 15. Because these components typically have tolerances of±5% and ±20%, respectively, the overall accuracy of sourcing currentmeasurements is approximately ±20%. Thus, the present invention providessourcing current measurements that are at least four times more accuratethan those provided by conventional systems.

The next step in calculating modulation current I_(M) is to measure thevoltage drop across modulation resistor 50 and compare it to the voltagedrop across sensing resistor 15. A/D converter 55 accomplishes this andobtains a value for X_(MS), which is defined by the following:X _(MS) =V _(M) /V _(S)  (6)where V_(M) is the voltage across modulation resistor 50 and V_(S) isthe voltage across sensing resistor 15. According to Ohm's Law, each ofthese voltages ate equal to the product of their respective current andresistance. Therefore:V _(M) =I _(M) *R _(M); andV _(S) =I _(S) *R _(S)  (8)

Substituting equations 7 and 8 into equation 6 yields:X _(MS)=(I _(M) *R _(M))/(I _(S) *R _(S))  (9)

Solving equation 9 for modulation current I_(M) results in the followingrelationship:I _(M)=(X _(MS))*(R _(S) /R _(M))*(I _(S))  (10)

To solve equation 10, a value of I_(S) must be obtained. By substitutingequation 4 into equation 10, the following expression is derived:I _(M)=(X _(MS) *R _(S) /R _(M))*(X _(TR) *V _(REF) /R _(T))  (11)rearranging equation 11 to group like terms, the following is obtained:I _(M)=(X _(MS) *X _(TR))(R _(S) /R _(M))(V _(REF) /R _(T))  (12)

Thus, it can be seen from equation 12, that the measurement ofmodulation current I_(M) depends only on the reference voltage(V_(REF)), termination resistor 25, and the ratio of modulation resistor50 to sensing resistor 15. Because existing integrated circuitfabrication techniques permit the variance of the resistor ratio to beless than ±1%, the overall accuracy of the modulation currentcalculation remains approximately ±5%, without the use of trimming. Thisis a significant improvement over conventional measurement techniqueswhich yield an overall accuracy of about ±20%.

Although driver circuit 100 may be used in a wide variety ofapplications that involve signal modulation, it is particularly usefulfor light-based modulation systems. In an optical communication system,for example, it is often necessary to quickly toggle a light source,such as a laser diode, to produce light pulses that are used ascommunication signals. To do this efficiently, circuit designers usuallybias the light source such that it varies between two precise levels.One of the levels represents an optical “logic low,” during which aminimum optical signal is produced. The other level represents anoptical “logic high” which produces an optical signal of sufficientstrength to be differentiated from an optical “logic low” signal. Theshorter this dynamic range is, the faster the light source can be variedbetween a “logic low” and a “logic high.” By having accuratemeasurements of bias and modulation currents, the driver circuitry candetermine and optimize the extinction ratio (i.e., the amount of opticalpower need to produce a logic “high” signal compared to the opticalpower needed to produce a logic “low”) of the light source.

Accurately determining the operating currents and extinction ratio ofcircuit 100 has many advantages. For example, the average currentrequired to maintain a constant average optical output power provides agood indication of the operational condition of the light source. Forexample, the turn ON threshold of a laser diode tends to degrade overtime. By monitoring the average laser current, a communications systemmay accurately predict the imminent failure of the light source andprovide a warning about impending failure so replacement can be madewithout loss of service.

Moreover, accurate measurements of the modulation and bias currents areuseful for statistical process control during the final assembly oflaser transceiver modules, where accurate measurement of laser setupconditions may give an early indication of manufacturing problems.

Another embodiment of the present invention may be used to calculate themodulation current I_(M) that also yields an overall accuracy of ±5%.This approach is illustrated in FIG. 2 as driver circuit 200 which issimilar to driver circuit 100 with the exception that circuit 200employs a third A/D converter 60. A/D converter 60 measures the voltagedrop across sensing resistor 15 with respect to a reference voltage (notshown) and generates a value represented as X_(SR). A/D converter 55 nolonger uses the voltage across sensing resistor 15 as a reference. Next,A/D converter 40 operates as described above and determines X_(TR). A/Dconverter 55 then determines X_(MR), instead of X_(MS), where X_(MR) isthe voltage drop across modulation resistor 50 measured with respect toa reference voltage (not shown) so that:X _(MR) =V _(M) /V _(REF)  (13)

Using Ohm's law it can be shown that:X _(MR) =I _(M) *R _(M) /V _(REF)  (14)

Solving for I_(M) yields:I _(M) =X _(MR) *V _(REF) /R _(M)  (15)

X_(SR) may be obtained with A/D converter 60 and expressed using thefollowing relationship:X _(SR) =I _(S) *R _(S) /V _(REF)  (16)

Combining equations 3, 14, and 16 and rearranging gives the following:I _(M) =X _(MR)*(X _(TR) /X _(SR))*(R _(S) /R _(M))*(V _(REF))/R_(T)  (17)

Thus, it can be seen from equation 17, that the measurement ofmodulation current I_(M) depends only on the reference voltage(V_(REF)), termination resistor 25, and the ratio of modulation resistor50 to sensing resistor 15. The readings obtained from A/D converters 40,55, and 60 may then be sent to processing circuitry to perform thecalculations as described above (not shown). Bias and modulation currentinformation may be used to compute feedback information and theextinction ratio. These values may also be used internally by the drivercircuit or externally for statistical analysis.

Persons skilled in the art will recognize that the present invention maybe implemented using a variety of circuit configurations other thanthose shown and discussed above. For example, the present invention mayemploy some trimming on the internal reference voltage to improve itsaccuracy (e.g., to about ±1%). This further improves the accuracy ofcurrent measurements.

Moreover, termination resistor 25 need not be external to the integratedcircuit, rather it may be fabricated on the integrated circuit andtrimmed to the appropriate value. Furthermore, sinking circuit 45 may bereplaced by or used in conjunction with a signal source thatperiodically impedes and allows current flow, thus providing modulation.

The present invention may also use data collecting components other thanthe A/D converters depicted in FIGS. 1 and 2 to implement differentembodiments of the present invention. In addition, it will beappreciated that some or all of the resistors shown may be replaced byelements such as synthetic circuit components with impedances having areactive component rather than elements with purely resistive attributes(e.g., a switched capacitor circuit). All such modifications will berecognized as within the scope of the present invention, which islimited only by the claims that follow.

1-35. (canceled)
 36. A system comprising: an integrated circuit; a firstimpedance disposed on the integrated circuit; a second impedancedisposed on the integrated circuit; a first measurement device coupledto the first impedance configured to measure a first voltage drop acrossthe first impedance; a second measurement device coupled to the secondimpedance configured to measure a second voltage drop across the secondimpedance; a third impedance disposed outside of the integrated circuit;a third measurement device coupled to the third impedance configured tomeasure a third voltage drop across the third impedance; and processingcircuitry that utilizes information from the first, second, and thirdmeasurement devices to determine a current supplied by the integratedcircuit.
 37. The circuit of claim 36 wherein the first measurementdevice is an analog to digital converter.
 38. The circuit of claim 36wherein the second measurement device is an analog to digital converter.39. The circuit of claim 38 wherein the analog to digital converterfurther comprises a trimmed voltage reference.
 40. The circuit of claim36 wherein the third measurement device is an analog to digitalconverter.
 41. The circuit of claim 36 wherein the third impedance is aprecision resistor.
 42. The circuit of claim 36 wherein the thirdimpedance is a switched capacitor circuit.
 43. The circuit of claim 36wherein the third impedance is an external resistor.
 44. The circuit ofclaim 36 further comprising a sinking circuit coupled to the secondimpedance.
 45. A system comprising: an integrated circuit; a firstresistive element disposed on the integrated circuit; a second resistiveelement disposed outside of the integrated circuit; a first measurementdevice coupled to the first resistive element configured to measure afirst voltage drop across the first resistive element; a secondmeasurement device coupled to the second resistive element configured tomeasure a second voltage drop across the second resistive element; andprocessing circuitry that utilizes information from the first and secondmeasurement devices to determine a current supplied by the integratedcircuit.
 46. The circuit of claim 45 wherein the first measurementdevice is an analog to digital converter.
 47. The circuit of claim 45wherein the second measurement device is an analog to digital converter.48. The circuit of claim 47 wherein the analog to digital converterfurther comprises a trimmed voltage reference.
 49. The circuit of claim45 wherein the second resistive element is a precision resistor.
 50. Thecircuit of claim 45 wherein the second resistive element is a switchedcapacitor circuit.
 51. The circuit of claim 45 wherein the secondresistive element is an external resistor.
 52. The circuit of claim 45further comprising a sinking circuit coupled to the first resistiveelement.
 53. A system comprising: an integrated circuit; a referencevoltage source; a first measurement device configured to measure voltagelevels across two impedance elements located on the integrated circuitrelative to the reference voltage source; a second measurement deviceconfigured to measure the voltage level across an impedance elementexternal to the integrated circuit relative to the reference voltagesource; and processing circuitry that utilizes information from thefirst and second measurement devices to determine a current supplied bythe integrated circuit.
 54. The circuit of claim 53 wherein the firstmeasurement device is an analog to digital converter.
 55. The circuit ofclaim 53 wherein the second measurement device is an analog to digitalconverter.
 56. The circuit of claim 55 wherein the analog to digitalconverter further comprises a trimmed voltage reference.
 57. The circuitof claim 53 wherein the impedance element external to the integratedcircuit is a precision resistor.
 58. The circuit of claim 53 wherein theimpedance element external to the integrated circuit is a switchedcapacitor circuit.
 59. The circuit of claim 53 wherein the impedanceelement external to the integrated circuit is an external resistor. 60.The circuit of claim 53 further comprising a sinking circuit coupled oneof the two impedance elements on the integrated circuit.
 61. The circuitof claim 53 wherein the supplied current is a modulation current.
 62. Amethod comprising: providing an integrated circuit; providing a firstimpedance disposed on the integrated circuit; providing a secondimpedance disposed on the integrated circuit; measuring a first voltagedrop across the first impedance; measuring a second voltage drop acrossthe second impedance; providing a third impedance disposed outside ofthe integrated circuit; measuring a third voltage drop across the thirdimpedance; and utilizing the first, second, and third measurements tocalculate a supplied current from the integrated circuit.
 63. The methodof claim 62 wherein the first measurement provides an analog to digitalconversion.
 64. The method of claim 62 wherein the second measurementprovides an analog to digital conversion.
 65. The method of claim 62wherein the analog to digital conversion further comprises a trimmedvoltage reference.
 66. The method of claim 62 wherein the thirdmeasurement provides an analog to digital conversion.
 67. The method ofclaim 62 wherein the third impedance is a precision resistor.
 68. Themethod of claim 62 wherein the third impedance is a switched capacitorcircuit.
 69. The method of claim 62 wherein the third impedance is anexternal resistor.